1. Field of the Invention
The present invention relates to an optical information recording medium that allows information signals to be recorded/reproduced with high quality by irradiating a thin film formed on a substrate with a high energy beam such as a laser beam, a method for producing the same, and a method and an apparatus for recording/reproducing information thereon.
2. Description of the Prior Art
Conventionally, it is known that it is possible to cause a phase change between an amorphous phase and a crystalline phase, which have different optical constants (refractive index n and extinction coefficient k), by irradiating a thin film made of a chalcogen material formed on a substrate with a laser light beam for local heating under different irradiation conditions. Utilizing this phenomenon, a so-called phase changeable optical information recording medium has been under development.
In the phase changeable optical information recording medium, a new signal can be recorded while erasing an existing recorded signal, using only a single laser beam, by modulating the laser output between two levels of a recording level and an erasing level in accordance with the information signal and irradiating an information track with the single laser beam. This method does not require a magnetic circuit component as required by optical magnetic recording. Therefore, this method is advantageous for recording information in that the head can be simplified and erasure and recording can be performed simultaneously so that a period of time required for rewriting can be shortened.
In such an optical information recording medium, the following structure is common. Dielectric layers having excellent heat resistance are provided as protective layers above and below a recording layer for the purpose of preventing the evaporation of the recording layer and the thermal deformation of the substrate that might occur during repeated use. Furthermore, a reflective layer made of a metal material is provided on the protective layer on the side opposite to the substrate for the purpose of efficiently using incident light and increasing the cooling rate so as to facilitate a change to an amorphous state. Thus, in general, at least four thin films are laminated to form the optical information recording medium.
In order to produce a phase changeable optical information recording medium with high density and large capacity, the following attempts are commonly carried out: forming a smaller mark by using a shorter wavelength of the light source or a higher NA (numerical aperture) of the object lens used for recording, and thus improving the linear density in the circumferential direction and the track density in the radial direction of the recorded mark on the substrate. Furthermore, mark edge recording in which information is defined by the length of the mark has been proposed to improve the linear density, and land and groove recording in which information is recorded both on grooves for guiding laser light formed on the substrate and lands between the guide grooves has been proposed to improve the track density, and both recording methods are adopted.
Furthermore, it was proposed that a plurality of such recordable information layers are laminated via separating layers so that the capacity is increased (e.g., JP 9-212917A). Moreover, layer recognition means and layer switching means for selecting one of these information layers for recording and reproduction were proposed (e.g. JP10-505188 A).
Improving not only high density but also data processing rates, namely, the velocity of recording/reproducing information, is important. For this reason, improving the linear velocity by rotating a disk at a higher revolution per minute with the radius position unchanged for recording and reproduction is under research.
In the case of overwriting with a single beam, the amorphous portion and the crystalline portion have different end-point temperatures when they are irradiated with beams of the same power level, because the amorphous portion and the crystalline portion have different light absorptances, and the crystalline portion requires a latent heat of fusion. Therefore, when overwriting a signal, the shape of the mark is distorted by the influence of a signal that has been recorded before the overwriting. This mark distortion causes an increase of errors (jitters) in the time axis direction of reproducing signals or a drop of the erasure ratio. The problem caused by this phenomenon becomes more serious as higher linear velocity and higher density are achieved.
In order to solve this problem, a method of equalizing the end-point temperature of the amorphous and crystalline portions irradiated with beams of the same power level was proposed (e.g., JP 1-149238A). This method requires that the absorptance ratio Ac/Aa is more than 1.0, where Ac is the absorptance of the crystalline portion, and Aa is the absorptance of the amorphous portion with respect to a laser light beam of wavelength xcex, in order to compensate the latent heat of fusion in the crystalline portion. In addition, when Rc is the reflectance of the crystalline portion, and Ra is the reflectance of the amorphous portion with respect to a laser light beam of wavelength xcex, the larger absolute value of the difference in the reflectance xcex94R=Rcxe2x88x92Ra is more desirable for larger signal amplitudes and higher C/N ratios.
There are two ways of increasing the absolute value of xcex94R, namely, a reflectance-decrease-type in which xcex94R is positive and a reflectance-increase-type in which xcex94R is negative. In the reflectance decrease-type, Rc can be raised easily, so that the reflectance as the base can be raised, and Ra can be substantially 0. Therefore, this is advantageous in that the contrast of a signal can be large. On the other hand, as described above, either one of the following is necessary in order to increase Ac/Aa at the same time: transmitting part of the incident light or allowing light to be absorbed by a portion other than the recording layer. This is disadvantageous in efficiently utilizing the incident light and in the freedom degree in the optical design. On the other hand, in the reflectance-increase-type, Ac/Aa can be increased at the same time when the absolute value of xcex94R is increased. Therefore, it is not necessary to transmit part of the incident light or to allow light to be absorbed by a portion other than the recording layer. This is advantageous in efficiently utilizing the incident light and in the freedom degree in the optical design.
Examples of the structure of such a reflectance-increase-type recording medium are as follows: A structure is such that at least five layers of a semitransparent optical interference layer made of Au or the like, a lower protective layer, a recording layer, an upper protective layer and a reflective layer are formed in this order on a substrate, and the absolute value of xcex94R is increased by the reflectance-increase-type technique utilizing the interference effect of light, especially by the optical interference layer (e.g., JP 7-78354A, JP 7-105574A and JP 7-262607A); and another structure is such that at least six layers of a protective layer with a high refractive index, a protective layer with a low refractive index, a protective layer with a high refractive index, a recording layer, an upper protective layer and a reflective layer are formed in this order on a substrate.
In the conventional reflectance-increase-type recording medium, a metal material such as Au and Al or an alloy material based on these metals is used as the reflective layer. All of these reflective layer materials have an refractive index n of less than 2.5, an extinction coefficient k of 3 or more, and a heat conductivity of more than 50 W/(mxc2x7K), so that they are classified in the class of thin film materials having a high heat conductivity. Therefore, since the cooling effect by the reflective layer is too large, the laser diodes that are available at the moment have insufficient power, so that the recording layer cannot be warmed sufficiently. Thus, a mark cannot be formed completely in some cases (insufficient sensitivity). Alternatively, thermal diffusion becomes large in the plane of the film during recording, so that a mark in an adjacent track may be erased (cross erase).
As described above, in the recording medium in which a plurality of information layers are laminated via separating layers for larger capacity, the insufficient sensitivity problem is even more serious, because recording/reproducing information in the second and subsequent information layers from the side to which laser light for recording/reproduction is incident is performed with the quantity of light that has been reduced by the reflection and/or the absorption when the light passes through the first information layer.
Furthermore, when the laser light has a short wavelength for high density recording, the output from the laser light source tends to be reduced. For this reason, the insufficient sensitivity may constitute a problem also in a medium including a single information layer.
Therefore, with the foregoing in mind, it is an object of the present invention to provide an optical information recording medium with high sensitivity while achieving a C/N ratio for high density and high linear velocity for overwriting, a high erasure ratio, and reduced cross erase. It is another object of the present invention to provide a method for producing such an optical information recording medium, a method for recording/reproducing information thereon, and an apparatus for recording/reproducing information thereon.
In order to achieve the above object, a first optical information recording medium of a preferable embodiment of the present invention includes a first information layer, a separating layer, a second information layer and a protective substrate in this order on a transparent substrate, wherein the second information layer comprises an optical interference layer, a lower protective layer, a recording layer that changes between two different states detectable optically by irradiation of light beams, an upper protective layer, and a reflective layer whose refractive index is at least 2.5 at a wavelength xcex of light beams used for recording in this order from the side near the transparent substrate, and the reflectance of the second information layer with respect to the light beams used for recording incident from the transparent substrate side after recording is higher than that before recording.
This optical information recording medium belongs to the category of a so-called reflectance-increase type and is provided with the optical interference layer in addition to the reflective layer. The optical interference layer serves to enlarge the increase of the reflectance (difference in the reflectance before and after recording) with optical interference.
More specifically, it is preferable that the refractive index of the optical interference layer at a wavelength xcex is not less than 2, and the extinction coefficient thereof is not more than 2, or that the refractive index of the optical interference layer at a wavelength xcex is not more than 1, and the extinction coefficient thereof is not less than 3.
Furthermore, it is preferable that the heat conductivity of the optical interference layer is not more than 50 W/(mxc2x7K). This is advantageous for improving the sensitivity.
The above optical information recording medium can include a first optical interference layer and a second optical interference layer in this order from the side near the transparent substrate. In this case, it is preferable that the refractive index of the second optical interference layer at a wavelength xcex is smaller than that of the first optical interference layer and is smaller than that of the lower protective layer, and that the extinction coefficient of the first optical interference layer and the extinction coefficient of the second optical interference layer are both not more than 1.
Furthermore, a second optical information recording medium of a preferable embodiment of the present invention includes a first information layer, a separating layer, a second information layer and a protective substrate in this order on a transparent substrate, wherein the second information layer comprises a lower protective layer, a recording layer that changes between two different states detectable optically by irradiation of light beams, an upper protective layer, and a reflective layer whose refractive index is at least 2.5 at a wavelength xcex of light beams used for recording in this order from the side near the transparent substrate. The lower protective layer is formed directly on the separating layer. The ratio n1/n2 of the refractive index n1 at the wavelength xcex of the recording layer before recording to the refractive index n2 thereof after recording is not more than 0.8. The reflectance after recording of the second information layer with respect to the light beams used for recording incident from the transparent substrate side is higher than that before recording.
Although the above optical information recording medium belongs to the category of the so-called reflectance-increase type, the optical interference layer is not present between the separating layer and the lower protective layer. In this medium, the recording layer having a ratio n1/n2 of the refractive index n1 at the wavelength xcex before recording to the refractive index n2 thereof after recording of not more than 0.8 is used instead of using the optical interference layer.
In the first and second optical information recording media, it is preferable that the first information layer comprises a recording layer that changes between two different states detectable optically by irradiation of light beams, and the first information layer has a transmittance of at least 30% with respect to the light beams used for recording. This is advantageous for improving the sensitivity.
The recording layer in the second optical information recording medium can be used not only for a medium provided with the two information layers, but in a wide range of applications. A third optical information recording medium including this recording layer of the present invention includes a lower protective layer, a recording layer that changes between two different states detectable optically by irradiation of light beams, an upper protective layer, and a reflective layer whose refractive index n is at least 2.5 at a wavelength xcex of light beams used for recording on the transparent substrate. The lower protective layer is formed directly on the transparent substrate. The ratio n1/n2 of a refractive index n1 at the wavelength xcex of the recording layer before recording to a refractive index n2 thereof after recording is not more than 0.8. The reflectance after recording of the medium with respect to the light beams used for recording incident from the transparent substrate side is higher than that before recording.
In the first, second and third optical information recording media of the present invention, it is preferable that the refractive index at the wavelength xcex of the reflective layer is not less than 3.0, and the extinction coefficient k at the wavelength xcex of the reflective layer is not more than 4.0.
Furthermore, it is preferable that the heat conductivity of the reflective layer is not more than 50 W/(mxc2x7K). In particular, when the heat conductivities of the optical interference layer and the reflective layer are both in the above range, the medium can be most suitable for improving the sensitivity.
Furthermore, it is preferable that the above optical information recording medium further includes an interface layer at at least one interface selected from the group consisting of an interface between the lower protective layer and the recording layer and an interface between the recording layer and the upper protective layer.
Furthermore, in the above optical information recording medium, it is preferable that the recording layer comprises Ge, Sb and Te.
Furthermore, in the above optical information recording medium, it is preferable that 0.10xe2x89xa6xxe2x89xa60.50 and 0.40xe2x89xa6zxe2x89xa60.60 are satisfied, wherein a ratio by number of atoms of Ge, Sb and Te is expressed by x:y:z (x+y+z=1).
Furthermore, in the above optical information recording medium, it is preferable that the reflective layer comprises at least one element selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Zn, Cd, Si, Ge, Sn, Pb, Sb, Bi, and Te.
In order to achieve the above object, a method for producing an optical information recording medium of the present invention is a method for producing the first or the second optical information recording medium, and includes forming the first information layer on the transparent substrate and forming the second information layer on the protective substrate; attaching the transparent substrate and the protective substrate via the separating layer in such a manner that the first information layer faces the second information layer; and initializing the first information layer and the second information layer so that the two layers are in a recordable initialized state.
In the above method for producing an optical information recording medium, it is preferable that the extinction coefficient of the reflective layer at a wavelength xcex of light beams is not more than 4.0, and the initialization is performed by irradiating the first information layer with the light from the transparent substrate side and irradiating the second information layer with the light from the protective substrate side, after the attachment process.
In order to achieve the above object, a method for recording/reproducing information on an optical information recording medium of the present invention is a method of recording/reproducing information on the first or second optical information recording medium, and is characterized in that information is recorded/reproduced on the first information layer and the second information layer with the light beams incident from the transparent substrate side.
In the above method for recording/reproducing information on an optical information recording medium, it is preferable that at least a part of a mark to be recorded is recorded on the recording layer with light beams with a pulse train including a plurality of pulses that are modulated between power level P1 and power level P3, and the light beams are kept constant at power level P2 when a mark is not formed (P1 greater than P2xe2x89xa7P3xe2x89xa70), where P1 is a power level sufficient to melt an irradiated portion of the recording layer instantly, and P2 and P3 are power levels that cannot melt the irradiated portion instantly.
Furthermore, in the above method for recording/reproducing information on an optical information recording medium, it is preferable that a cooling section of power level P4 is provided after a last pulse of a recording pulse train, where P2 greater than P4xe2x89xa70.
In order to achieve the above object, an apparatus for recording/reproducing information on an optical information recording medium of the present invention is an apparatus for recording/reproducing information on the first or second optical information recording medium, and includes layer recognition means and layer switching means for recording/reproducing information on the optical information recording medium, wherein the information is recorded/reproduced on the first information layer and the second information layer with the light beams incident from the transparent substrate side.
The above apparatus for recording/reproducing information on an optical information recording medium further includes light beam-intensity-modulating means for recording at least a part of a mark to be recorded on the recording layer with light beams with a pulse train including a plurality of pulses that are modulated between power level P1 and power level P3, while keeping the light beams constant at power level P2 when a mark is not formed (P1 greater than P2xe2x89xa7P3xe2x89xa70), where P1 is a power level sufficient to melt an irradiated portion of the recording layer instantly, and P2 and P3 are power levels that cannot melt the irradiated portion instantly.
Furthermore, it is preferable that the light beam-intensity-modulating means provides a cooling section of power level P4 after a last pulse of a recording pulse train, where P2 greater than P4xe2x89xa70.
As described above, the present invention provides an optical information recording medium having a high C/N ratio, a high erasure ratio and high sensitivity for overwriting in high density and at high linear velocity, and having reduced cross erase and a large capacity.